The present disclosure relates to an apparatus and method for diagnosing a fuel cell, and more particularly to a technology for diagnosing a state of a fuel cell by estimating a fuel-cell equivalent circuit.
A fuel cell stack is an energy conversion system that generates electrochemical reaction by applying fuel gas and oxygen gas to a membrane electrode assembly (MEA) so that chemical energy is converted into electric energy.
A solid polymer electrolyte fuel cell stack designed to use a solid polymer film as an electrolyte is inexpensive and has a compact structure, and has high output density, so that the solid polymer electrolyte fuel cell has been widely used as a vehicle-embedded power source.
Throughput and lifetime a Polymer Electrolyte Membrane Fuel Cell (PEMFC) (hereinafter referred to as a fuel cell) are affected by operation conditions of the fuel cell. Operation conditions of the fuel cell may include current, temperature, amount of reactant, pressure of reactant, amount of cooling material, the amount of moisture content, etc.
In order to optimally control the operation conditions of the above-mentioned fuel cell on the basis of a current state of the fuel cell, many developers and companies are conducting intensive research into various methods for diagnosing the fuel cell state.
Representative examples for diagnosing the above fuel-cell state may include AC (Alternating Current) impedance measurement, current-voltage curve measurement, catalytic-area measurement, etc. so as to diagnose a state of the above fuel cell.
In this case, the AC impedance measurement generally inputs an AC signal (voltage or current) of frequency f of 0.1˜1000 Hz to a fuel cell, measures a response in each region, and thus calculates impedance. The impedance measurement has been widely used to recognize an internal state of the fuel cell.
That is, an alternating current is applied to the fuel cell, and an AC voltage of the fuel cell is measured with respect to the alternating current. Impedance Z(f) of the fuel cell is calculated. In this case, the impedance of the fuel cell may be represented by “Z(f)=Vac(f)/Iac(f)”. The internal state of the fuel cell is recognized through the calculated fuel-cell impedance Z(f).
The following effects can be obtained by measuring the impedance of the fuel cell.
For example, impedance of the alternating current of 300 Hz is measured so that the amount of moisture contained in the fuel cell can be measured. That is, the moisture content of the fuel cell is inversely proportional to output impedance.
Impedance of the 20 Hz alternating current is measured so that an internal state of the fuel cell is measured. In addition, impedance of the 4 Hz alternating current is measured so that a state of the gas supply to the fuel cell is measured. If the gas supply to the fuel cell is not facilitated, impedance of the fuel cell is increased and the output and operation stability of the fuel cell are deteriorated.
However, the above-mentioned method for diagnosing the fuel cell state on the basis of an impedance value measured by the fuel cell has difficulty in recognizing a correct state of the fuel cell. That is, the conventional fuel-cell-state measurement method recognizes the internal moisture state of the fuel cell using an impedance value of one frequency (e.g., 300 Hz), uses data of all the obtained frequency ranges, and measures impedance of one fixed fuel-cell equivalent circuit model.
Since the method for recognizing an internal state of the fuel cell using impedance of one frequency is easily affected by measurement error and noise, it has difficulty in reliably diagnosing the fuel cell state.
Assuming that the fuel cell state is diagnosed through the equivalent circuit model, the equivalent circuit model has the same degree of freedom as in the number of parameters, so that a complicated model requires a huge number of calculations. Therefore, a super-high-capacity memory and a higher calculation capability are needed such that production costs and power consumption are unavoidably increased.
The method for deriving the fuel-cell equivalent circuit parameter using Complex Non-linear Least Squares (CNLS) does not always obtain an optimum value, and may derive a local solution instead of a global solution as necessary.
However, there is a high possibility of deriving parameter values quite different from the actual internal state of the fuel cell. The above-mentioned local solution may have a higher possibility of deriving parameter values as the given model has higher complexity.